Infrared detector arrays for satellite use are well known. Current infrared detection systems incorporate arrays of large numbers of discrete, highly sensitive detector elements, the outputs of which are connected to sophisticated processing circuitry both on the focal plane and in an on-board data processor. The on-board data processor rapidly analyzes the pattern and sequence of detector element excitations to identify and monitor sources of infrared radiation.
A contemporary subarray of detectors may contain 256 detectors on a side, or a total of 65,536 detectors. The size of each square detector is approximately 0.009 cm on a side with 0.00127 cm spacing between detectors. Such a subarray would therefore be 2.601 cm on a side. The subarray may, in turn, be joined to form an array that connects 25,000,000 or more detectors.
It is desirable to keep the array scan cycle as short as possible to maintain the highest possible array frame scan rate. This provides the highest possible number of frames per minute. The higher the array frame scan rate, the quicker the image data is updated and the sooner any change will be detected.
In the contemporary infrared detector element arrays the scan cycle is divided into two equal portions. During the first portion, called the charge accumulation interval, the outputs of dedicated detector elements are simultaneously accumulated upon a plurality of capacitors and stored as charges. During the second portion of the scan cycle, called the output interval, signals representative of these stored charges are output to external processing circuitry.
It is desirable to increase the interrogation time or charge integration interval to the maximum time interval possible to improve the signal-to-noise ratio of the detector element output. The signal-to-noise ratio is improved by increasing interrogation time because transient output fluctuations due to noise become a lower proportion of the total output signal as the amount of the total output signal is increased. Fluctuations due to noise also tend to cancel over time, therefore making the integration of the current output of the detector elements over a comparatively long time interval a desirable method for improving the signal-to-noise ratio of the detector element outputs.
The shorter the interrogation time, the greater the effect of any variation in the output signal due to noise. For instance, if the interrogation time were limited to the time interval during which a noise spike occurred, the output signal would consist substantially of the noise spike and would consequently provide erroneous data. However, if that same noise spike occurred at some point during a much larger interval of time and the output of the detector element was averaged or integrated over that larger interval of time, then the effect of the noise spike would be proportionally reduced and the data thus provided would be much more useful.
An erroneous reading can also occur when a change in detector output occurs during the charge accumulation interval. A change in detector output occurs when the detected image changes, such as when an object moves into the field of view.
The inherent characteristics of the detector element and its associated circuitry cause the detector output signal to rise with a finite slope when a detector element is excited. This slope typically occurs over an interval of time greater than the charge accumulation interval of contemporary infrared detector arrays. The interrogation of a detector element during a period of time when the output signal is rising, for example, may either be interpreted by the processing circuitry, i.e. the analog to digital converter, as an unchanged output or as an increased output depending upon the amount by which the detector output has increased. The result depends upon exactly when the detector element is interrogated. Therefore, it is possible for an increase in detector output to be interpreted by the processing circuitry as either no change or an increased output when the actual level of the detector is increasing. This is possible because of the discrete quantization involved in the analog to digital conversion process.
It would be desirable to increase the interrogation time or charge accumulation interval of the detector elements without reducing the frame scan rate. This would provide a more reliable interpretation of the detector element signals without reducing the rate at which information is processed. It would result in a more complete and reliable representation of objects within the detector element field of view. It would also allow the on-board signal processor to recognize a change in the field of view of the detector element array at the earliest possible time.
As such, although the prior art has recognized the need for increased detector element interrogation time to improve the signal-to-noise ratio and to improve the reliability of the digitization process without reducing the frame scan rate, the problem has heretofore never been addressed.